Standards for the Long-Term Evolution (LTE) wireless network have been developed by members of the 3rd-Generation Partnership Project (3GPP). The members of 3GPP are currently developing the Release 11 specifications for LTE. These developing standards will include specifications for yet another technology for extending high throughput coverage, namely improved support for Coordinated Multipoint (CoMP) transmission/reception, where multiple nodes coordinate transmissions and receptions to increase received signal quality and reduce interference.
CoMP transmission and reception refers to techniques used in a wireless communication system in which the transmission and/or reception at multiple, geographically separated antenna sites is coordinated, to improve system performance. More specifically, the term CoMP refers to the coordination of antenna arrays that have different geographical coverage areas. In the discussion that follows, an antenna or group of antennas covering a certain geographical area is referred to as a point, or more specifically as a Transmission Point (TP). The coordination and control of multiple transmission points can either be distributed, by means of direct communication between the different sites, or by means of a central coordinating node.
CoMP has been introduced into LTE to improve the coverage of high-data-rate services, to increase cell-edge throughput, and/or to increase system throughput. In particular, the goal is to distribute the user-perceived performance more evenly in the network by taking better control of the interference. CoMP operation targets many different deployments, including coordination between sites and sectors in cellular macro deployments, as well as coordination in various configurations of heterogeneous deployments, where, for instance, a macro node coordinates its transmission with pico nodes within the macro coverage area.
There are many different CoMP transmission schemes that are being considered and/or developed. For example, one approach is called Dynamic Point Blanking, where multiple transmission points coordinate transmissions so that a neighboring transmission point may mute transmissions on the specific time-frequency resources that are allocated to mobile terminals (“UEs,” in 3GPP parlance) and that are experiencing significant interference. Another approach is called Dynamic Point Selection, where the data transmission to a UE may switch dynamically (in time and frequency) between different transmission points, so that the transmission points are fully utilized. In another approach, Coordinated Beamforming, the transmission points coordinate transmissions in the spatial domain by beaming the transmission power in such a way that the interference to UEs served by neighboring transmission points is suppressed. With Joint Transmission, a given transmission to a UE is simultaneously transmitted from multiple transmission points, using the same time-frequency resources.
One common denominator for the various CoMP transmission schemes is that the network needs channel-state information (CSI) not only for the serving transmission point, but also for the radio channels linking neighboring transmission points to a mobile terminal. For that purpose the CoMP Measurement Set is introduced in LTE. The underlying basis of the CoMP Measurement Set is a new reference symbol sequence, the CSI-RS, which was introduced in LTE Release-10 specifications for the express purpose of estimating channel state information. The CoMP Measurement Set enables the eNodeB to configure a set of CSI-RS resources that the UE will use to perform channel measurements for providing CSI feedback to the network. A CSI-RS resource, which generally corresponds to a particular transmission point, can loosely be described as a pattern of time-frequency resource elements on which a particular CSI-RS configuration is transmitted. A CSI-RS resource is determined by a combination of the LTE parameters “resourceConfig”, “subframeConfig”, and “antennaPortsCount”, which are configured by Radio Resource Control (RRC) signaling.
For LTE, 3GPP has adopted an implicit CSI mechanism for CSI feedback. With this approach, a UE does not explicitly report the complex valued elements of a measured effective channel, for example, but instead recommends a transmission configuration for the measured effective channel. The recommended transmission configuration thus implicitly gives information about the underlying channel state.
In Release 8 and 9 of LTE the CSI feedback is given in terms of a transmission rank indicator (RI), a pre coder matrix indicator (PMI), and channel quality indicator(s) (CQI). The CQI/RI/PMI report can be wideband or frequency selective depending on which reporting mode that is configured. The RI corresponds to a recommended number of streams that are to be spatially multiplexed and thus transmitted in parallel over the effective channel. The PMI identifies a recommended precoder (in a codebook) for the transmission, which relates to the spatial characteristics of the effective channel. The CQI represents a recommended transport block size (i.e., code rate). There is thus a relation between a reported CQI and a signal-to-interference-plus-noise ratio (SINR) of the spatial stream(s) over which the transport block is transmitted. It will be appreciated that in some contexts a CQI is understood to refer to a SINR or a similar received signal quality metric, but that is not the proper definition in LTE contexts. Most notably, reporting an SINR corresponds to the category of explicit CSI, whereas CQI as defined above fall in the implicit CSI category.
The implicit feedback framework has many advantages over more explicit feedback. Most notably, the UE implementation becomes to a large extent transparent to the reporting mechanism and the testing thereof. Further, the implicit feedback framework encourages advanced or particularly effective receiver implementation, since high-performing UEs can report higher CQI and/or higher transmission rank and thus immediately benefit from the added implementation effort. Advanced receiver designs may include, for example, the use of an increased number of UE receive antennas, advanced interference suppression techniques, and/or advanced channel estimation for demodulation and CSI reporting.
Explicit CSI feedback, on the other hand, has the disadvantage that possible benefits from the reporting UE's receiver implementation are typically not reflected in the reporting, and it therefore becomes increasingly difficult for the network and UE to manage and fully exploit different UE receiver implementations. Moreover, it is generally more difficult to provide effective interoperability testing for such CSI feedback mechanisms.
The use of CSI-RS for CSI reporting provides several advantages over basing the CSI feedback on the common reference symbols (CRS), which were used for that purpose in previous releases. First, the CSI-RS is not used for demodulation of the data signal, and thus does not require the same density. In other words, the overhead consumed by CSI-RS is substantially less than is required by reference signals that are used to provide a reference for signal demodulation. Second, CSI-RS provides a much more flexible means to configure CSI feedback measurements, in that which CSI-RS resource to measure on can be configured in a UE-specific manner. Third, CSI-RS can be used to generate CSI feedback for more than four antennas. In fact, the support of antenna configurations larger than 4 antennas must resort to CSI-RS, since CRS are only defined for at most 4 antennas.
Related to CSI-RS is the concept of zero-power CSI-RS resources, which are also known as muted CSI-RS. These are identified to the mobile terminal (i.e., “configured”) in the same way that regular CSI-RS resources are configured. A UE thus knows that data transmissions are mapped around both the zero-power CSI-RS resources and any CSI-RS resources configured for measurement. The intent of the zero-power CSI-RS resources is to enable the network to mute the transmission on the corresponding time-frequency resources so as to boost the SINR of a corresponding non-zero power CSI-RS, which might be transmitted, for example, from a neighbor cell and/or transmission point.
For Release 11 of LTE, a special zero-power CSI-RS that a UE is mandated to use for measuring interference-plus-noise is under discussion. As the name indicates, a UE can assume that the transmission points of interest are not transmitting on the muted CSI-RS resource and the received power can therefore be used as a measure of the interference-plus-noise level.
Based on a specified CSI-RS resource and an interference measurement configuration (e.g., a muted CSI-RS resource), the UE can estimate the effective channel and interference-plus-noise, and consequently also determine which rank, precoder and transport format to recommend that best match the particular channel.
As noted above, to support CoMP transmission the network needs CSI information not only for the serving transmission point but also for the channels linking the neighboring transmission point to a mobile terminal of interest. By configuring a unique CSI-RS resource per transmission point, for example, a UE can resolve the effective channels for each transmission point by measurements on the CSI-RS corresponding to that transmission point. Note that the UE is generally unaware of the physical presence of a particular transmission point; it is only configured to measure on a particular CSI-RS resource, without knowing of any association between the CSI-RS resource and a particular transmission point.
A few candidates for CoMP feedback techniques are on the table for LTE Release 11 specifications. Most alternatives are based on per CSI-RS resource feedback, possibly with CQI aggregation of multiple CSI-RS resources, and also possibly with some sort of co-phasing information between CSI-RS resources. The following list briefly introduces a few relevant alternatives. Note that a combination of one or more of these alternatives is also possible.
Per-CSI-RS resource feedback corresponds to separate reporting of channel state information (CSI) for each CSI-RS of a set of CSI-RS resources. Such a CSI report could, for example, correspond to a Precoder Matrix Indicator (PMI), a Rank Indicator (RI), and/or a Channel Quality Indicator (CQI), any or all of which represent a recommended configuration for a hypothetical downlink transmission over the same antennas used for the associated CSI-RS (or other RS used for the channel measurement). More generally, the recommended transmission configuration should be mapped to physical antennas in the same way as the reference symbols used for the CSI channel measurement. There could be interdependencies between the CSI reports sent according to the per-CSI-RS resource feedback approach; for example, they could be constrained to have the same RI.
The considered CSI-RS resources are configured by the eNodeB as the CoMP Measurement Set. Often there is a one-to-one mapping between a CSI-RS and a transmission point, in which case per-CSI-RS resource feedback corresponds to per-transmission point feedback; that is, a separate PMI/RI/CQI is reported for each TP.
Aggregate feedback corresponds to a CSI report for a composite channel that corresponds to an aggregation of multiple CSI-RS. For example, a joint PMI/RI/CQI can be recommended for a joint transmission over all antennas associated with the multiple CSI-RS. However, a joint search may be too computationally demanding for the UE, and a simplified form of aggregation is to evaluate an aggregate CQI and RI, which are combined with per-CSI-RS resource PMIs. Such a scheme also has the advantage that the aggregated feedback may share much information with a per-CSI-RS resource feedback. This is beneficial because many CoMP transmission schemes require per-CSI-RS resource feedback, and to enable eNodeB flexibility in dynamically selecting CoMP scheme, aggregated feedback would typically be transmitted in parallel with per0 CSI-RS resource feedback. To support coherent joint transmission, such per-CSI-RS resource PMIs can be augmented with co-phasing information enabling the eNodeB to rotate the per-CSI-RS resource PMIs so that the signals coherently combine at the receiver.
CoMP schemes under consideration employ downlink transmission using UE-specific demodulation reference symbols (DMRS), which were introduced in LTE release 9. The DMRS are transmitted intertwined with the data symbols and are subject to the same precoding, so as to make non-codebook-based precoding possible. In order to facilitate processing gains in the channel estimation, it is important to keep the precoder fixed for some interval in the frequency domain as well as in temporal domain. Furthermore the UE needs to be aware where these constant precoding intervals are located. For transmission mode 9 in LTE, these intervals are termed precoding resource block groups (PRG), and the resource blocks within a PRG are subject to physical resource block (PRB) bundling.
Based on feedback reports received from UEs in its coverage area, the LTE base station, known in 3GPP terminology as an evolved NodeB or eNodeB, needs to schedule the UEs in the resource time-frequency grid, coordinate the transmission points that are under the eNodeB's control, and perform link adaptation for each scheduled link. The link adaptation takes the feedback from the UEs into account, but it is common practice to adjust the reported CQI in order to compensate for imperfections in the feedback, uncertainties regarding interference measurements, etc.
Typically there is a UE-specific and dynamically adjusted back-off to the CQI that is applied when the eNodeB performs link adaptation. This means that if there are systematic errors or unpredictable uncertainties in the feedback CQI, the eNodeB must increase its back-off so that a target block error rate (BLER) is met. Because uncertainties translate to losses in system throughput, techniques for reducing these uncertainties are desirable.